PP Amplifier output impedance matching

[Kalee20]

In a pushpull output, does the transformer need to present an Impedance on its entire primary to match the individual R.a. of the valves used, or does it need to match the R.a. in each half of the winding?

Easy part first - in Class A push-pull, you need the transformer to match double what each individual valve would require.

So if a valve single-ended requires 6kΩ, then two valves in push-pull would require 12kΩ anode-anode. This is because you have the same current as a single valve, but double the voltage swing between the two anodes. (If you had connected them in parallel, you'd need half the individual or 3kΩ, makes sense because you'd have the same voltage swing as a single, but double the current).

I was stuck indoors yesterday, and did some experiments, based on what I've read in several books on the subject, or, what I understand of what I read, I should say.

The quote from "Kalee20" is the concise answer to my original question, its a more difficult subject to completely understand than I can manage at the moment......the valve wizard articles posted by Diabolical Artificer are the easiest texts I found to understand.

Thanks! When I first read your post, I was not sure of your knowledge - I have now got a better idea from the recent posts so hope that my subsequent bit did not overcomplicate things!

From a learning point of view, Class A push-pull is the one to understand before moving to Class AB, Class B. These are capable of really good results, understandable in general principle but it is hard to see what is the ideal load, because none of them are 'right', just different compromises.

Class A pentodes or tetrodes are the easiest because generally you do not need to worry about grid current. The valvewizard articles you mention, the treatment of the single-ended case is pretty good, I agree with you. The article could have described a theoretically ideal pentode, and how to load that (you can choose your own operating point as long as you under-run the valve, and then calculate the optimum load for your choice of quiescent Ia very simply), and then gone on to a few fiddle-factors for 'real', non-ideal valves. It didn't, but it is still good.

Luckily, valves are pretty tough electrically and if you get things a bit wrong they will not instantly die. Further, you can get encouraging results even if the load is 'wrong' by a factor of 2, and have a lot of fun in the process.